This invention relates to a compact heat exchanger and/or fluid mixing means which incorporates a series of plates having apertures which define a plurality of passages through which fluid may flow.
Compact heat exchangers are characterised by their high xe2x80x9carea densityxe2x80x9d which means that they have a high ratio of heat transfer surface to heat exchanger volume. Area density is typically greater than 300 m2/m3. and may be more than 700 m2/m3. Such heat exchangers are typically used to cool (or heat) process fluids.
One well known but expensive to manufacture type of heat exchanger is the so-called tube and shell heat exchanger. Essentially such heat exchangers consist of an exterior tubular shell through which run a number of longitudinally-extending smaller diameter tubes carrying one or more fluids. Other fluids, with which heat is to be exchanged, typically pass transversely across the heat exchanger such that heat is exchanged through the tube walls. A large number of tubes may be needed and they each have to be individually and accurately fixed/secured into a header plate at each end of the shell. In each case holes need to be drilled in the header plates very accurately to locate the tubes. High quality tested tubing then needs to be assembled into the plates and brazed or welded or mechanically-expanded into position. As the tubes are reduced in diameter to increase surfaces available for heat transfer and hence performance/compactness, the more difficult and expensive such configurations become to manufacture.
A second known type of heat exchanger is the so-called primary plate/secondary plate type exchanger in which a stack of plates is assembled, the stack having primary plates which directly separate two different fluid streams and secondary plates between adjacent primary plates. The secondary plates act as fins which add to the strength of structure and may be provided with perforations to provide additional flow paths for the fluids. The plates are usually bonded together by brazing but this may have the disadvantage of affecting the physical properties of the plates in the brazed regions or may introduce into the system, by means of the braze material, a potentially less satisfactory structure in terms of strength and corrosion resistance. It has been proposed to bond the plates together by diffusion bonding but a satisfactory construction that can withstand the high pressures involved has not been achieved and the fins may buckle during the bonding process.
It is an object of the present invention to provide an improved construction of this second type of heat exchanger which can be satisfactorily made by, for example, diffusion bonding or by brazing. It also aims to provide a heat exchanger construction which can also be readily adapted for use as a fluid mixing means, e.g. it can be used as a chemical reactor in which fluids which are to react together are mixed. Thus, where a reaction is exothermic, the invention may provide a means whereby the exothermic heat of reaction may be removed efficiently or, alternatively, it may be used to supply heat to an endothermic reaction. The products of the invention are also useful as fuel reformers and gas clean-up units associated with fuel cell technology.
Accordingly the present invention provides a heat exchanger or fluid mixing means comprising a bonded stack of plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates and the passageway(s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated plates by an intervening plate.
The intervening plate may be unperforated to provide complete separation of the passageways of the respective groups of plates. Such an intervening plate will be referred to below as a xe2x80x9cseparator platexe2x80x9d. Alternatively, as is described in more detail below, the intervening plate may contain holes positioned and sized to provide controlled mixing of the fluids in those passageways. Such an intervening plate will be referred to below as a mixing plate.
Each group of main perforated plates comprises at least two perforated plates but may contain three or more adjacent perforated plates as desired. A stack may, for example, comprise two or more groups of main perforated plates separated by intervening plates each group containing two perforated plates having their perforations aligned in rows.
The passageways formed by the rows of discrete channels across the plates may simply traverse across the plates once from one side to the other. However, in a first specific embodiment, the perforations at one or both ends of each row are shaped to turn their respective channels through an angle whereby the passageway defined by the channels continues in a different direction through the stack.
In a second specific embodiment two or more separate passageways are provided across a group of plates whereby streams of different fluids may flow parallel to each other in the same layer provided by said group of plates. This embodiment can provide improved temperature profiles across the plates and reduced thermal stress.
Because the plates are stacked with the main perforated plates of each group aligned with their perforations in parallel rows, it will be appreciated that the solid regions (i.e. ribs) of those plates between the rows of perforations are also aligned in parallel rows. As the perforated plates, therefore, are stacked one above each other, the parallel rips are aligned through the stack and hence this not only provides the discrete channels referred to above, it provides strength through the assembled stack whereby the pressures generated in the bonding, process can be withstood. The invention, therefore, provides a stack structure that can be bonded without the risk of the fins of the secondary plates collapsing under the pressures generated. The fins also provide the means of withstanding internal pressures in the operating streams.
The perforations may be of any desired shape but are preferably elongated slots. In the aforementioned first embodiment the slots at thee end of a row are preferably xe2x80x9cLxe2x80x9d or xe2x80x9cVxe2x80x9d shaped with the angle of the xe2x80x9cVxe2x80x9d being determined by the desired change of direction of the passageway.
The plates may be rectangular, square or circular for example or of any other preferred shape.
Where the plates are square or rectangular, each row of slots may extend from a first edge of the plate parallel to a second edge of the plate and for substantially the whole length of that second edge. It will be appreciated that a substantially unperforated edge or border will normally be required around the perimeter of the major faces of the plate to enable the plates of the stack to be bonded together and to provide pressure containment for the stream or streams. However, a completely unperforated border is not essential and slots in the border may be required for inlet and outlet means, for example. A plurality of rows of slots may, therefore, extend across the plate from the first edge towards the opposite, third, edge. In respect of the first embodiment described above, adjacent that opposite third edge the slots at the end of the row may xe2x80x9cLxe2x80x9d shaped whereby each row then extends at right angles to its original direction, i.e. extends parallel to the third edge. A second right angle turn may then be arranged whereby the rows of slots then extend back across the plate parallel to the first plurality of rows and so on.
Depending on the number and width of the rows in each plurality of rows and on the width of the plate, this change of direction can be repeated several times across the plate. Thus a passageway defined by at least a pair of perforated plates may extend backwards and forwards across the plates, i.e. a multi-pass arrangement.
Where the plates are circular the rows and passageways may extend from the outer perimeter as a segment of the circle towards the centre and then turn through an angle xe2x80x9cxcex1xe2x80x9d to extend back towards the perimeter and so on. The rows and passageways (and hence the slots) can narrow as they get closer to the centre and the number of segments and hence turns will, of course, be determined by xcex1xc2x0, e.g. where xcex1xc2x0=45xc2x0, there will be eight segments.
In one particular arrangement of the aforesaid second embodiment, a stack may be built up of one or more similar groups of plates, each group comprising an upper and a lower unperforated, or primary plate, a multi-passageway input layer in contact with one primary plate and a corresponding multi-passageway output layer in contact with the other primary plate, a centrally-disposed layer having at least one passageway for a first fluid and two or more transfer passageways for a fluid from each passageway of the input layer, a first auxiliary perforated plate lying between the input layer and the centrally-disposed layer and a second auxiliary perforated plate lying between the output layer and the centrally-disposed layer, the perforations in the first auxiliary perforated plate being positioned to transfer fluid from each passageway of the input layer to the corresponding transfer passageways in the centrally-disposed layer and the perforations in the second auxiliary perforated plate being positioned to transfer fluid from the transfer passageways to the corresponding passageways of the output layer. The centrally-disposed layer can conveniently be formed of a plurality of main perforated plates as described above, as can the input and output layers.
The perforations or slots are preferably photochemically etched through the plates by known means, although spark erosion, punching or any other suitable means may be used, if desired.
It will be appreciated that the slots in one plate of the group of main perforated plates must not correspond directly with those of its stacked adjacent main perforated plate or plates so that the non-perforated regions of the two plates do not completely coincide but must only overflap so that the flow channels defined by the plates of the group are not blocked. Thus, if as is preferred, some or all of the plates of a group are identical, they must be positioned relative to each other with an overlap at one edge so that the transverse solid regions or bars between adjacent slots of a row do not coincide and thereby form a barrier to flow along the channel. It will also be appreciated that the spacing of the transverse bars affects the heat transfer performance as the fluid(s) are constrained to flow over or under the bars. Thus the plates may be designed to enhance heat transfer without excessive pressure drop.
Each of the plurality of fluid channels forming an individual passageway may pass through the stack without any communication with another channel of the passageway. No mixing of fluid in those channels can, therefore, take place and the stack functions purely as a heat exchanger with fluids at different temperatures passing through different groups of perforated plates or passing through different passageways in the same group of perforated plates.
In another embodiment there is provided intercommunication at selected positions between the channels of a passageway. Thus cross-channels or vents may be etched or otherwise formed in the plates to provide access between adjacent channels. The vents may be formed at any desired position along the flow channels. Thus fluids flowing through separate channels may be mixed at pre-arranged positions on their journey through the passageways through the stack and this mixing may be employed to improve heat exchange capability.
Alternatively or additionally, inlets for a further fluid may be provided through the peripheral borders of the plates. Thus reactant may be introduced and mixed via the peripheral border inlets whereby the stack may be employed as a chemical reactor.
In another embodiment the invention provides a stack in which a fluid stream from one group of main perforated plates may be injected into a fluid stream in an adjacent group of main plates. Injection holes for this purpose are provided in an intervening mixing plate which separates the two groups of main perforated plates. So-called xe2x80x9cprocess intensificationxe2x80x9d can be achieved by this means, and any reaction caused by the injection of a first fluid into a second fluid can be controlled by the pressure differential between the two streams, the size, numbers and spacing of the injection holes and by sandwiching the second stream between the first stream and a coolant or heating stream, as appropriate.
The density of the slots, and hence of tie ribs or fins between each row of slots, may be varied, as required. Thus the number of slots per unit width or per unit length of a plate may be arranged to suit any particular flow/pressure drop/distribution change requirements.
The rows of slots may extend linearly across the plate but this is not essential and they may be arranged in other desired patterns, e.g. herringbone or chevron.
The plates may be provided at their edges with extensions, e.g. in the form of lugs to assist location of the plates in a stack. Such lugs may be designed to be broken off after the stack has been assembled, e.g. by etching partway through their thickness along a line where the lug joins the plate. Alternatively the extensions may fit together in the stack to provide, e.g. one or more tanks on the side faces of the stack. Each extension may, for example, be in the form of a flat loop, e.g. of semi-circular profile, providing an aperture at the edge of the plate, the apertures of adjacent plates forming the volume of the tank when the plates are stacked together. The loops may be attached to the plate not only at their ends but also across the aperture by means of narrow ligaments. The tanks so formed can each feed fluid, e.g. process fluid, coolant or reactant which is fed into the tanks, into the channels of one passageway. Thus a tank will be coterminous on the side of the stack with the height and width of the passageway, i.e. a group of channels, to be fed. Where the stacks are polygonal in plan, a tank may be provided on one or more of the side faces of the stack. Where the stacks are circular in plan, a number of tanks may be spaced around the perimeter as desired.
As indicated above, the stacks of the invention are particularly well adapted to provide multi-stream and multi-pass operation.
Plates used to form the products of the invention may also be provided with a hole, e.g. disposed centrally through each plate, whereby a stack of the plates has a centrally-disposed discrete passageway for a fluid stream through the stack. To compensate for the loss of flow area where such a central hole is provided, it is possible, where the plate is provided with integral tank loops, to extend the plate outwardly between adjacent loops.
The plates of a stack are preferably of the same material and are preferably thin sheets of metal, e.g. of 0.5 mm thickness or less. The material is preferably stainless steel but other metals, e.g. aluminium, copper or titanium or alloys thereof, may be used.
Inlet and outlet headers or manifolds for the different fluids may be secured to the stack after bonding together of the stack plates or, alternatively, may be formed from integral features on the plates.
As indicated above, the components of a stack may be bonded together by diffusion bonding or by brazing. Diffusion bonding, where possible, may be preferred but, in the case of aluminium, which is difficult to diffusion bond, brazing may be necessary. It is then preferable to clad the aluminium surfaces, e.g. by hot-roll pressure, bonding, with a suitable brazing alloy, in order to achieve satisfactory bonding by the brazing technique, although other means to provide the braze medium may be used, e.g. foil or vapour deposition.
The invention is particularly useful where it is desired to build up a large heat exchanger by bonding side by side a number of heat exchanger units. Each unit can be provided by a stack of plates of the invention. Each stack may, for illustration purposes only, be formed of plates of, say, 300 mm width by 1200 mm length and of the desired height depending on the thickness and number of plates. Several stacks can be placed side by side on a separator plate and then the assembly closed at the top by another separator plate. If six stacks, for example, are utilised side by side, a heat exchanger of about 1800 mm length is achieved. All required lugs, mitre sections, spacers, etc. can be formed integrally and built up from appropriate formations on each plate and all the stacks will be of the same height, being made up of identical plates. Such an arrangement has significant advantages in the manufacture of, for example, xe2x80x9ccryogenicxe2x80x9d aluminium heat exchangers, which conventionally have to be built up of layers of corrugations with separate side bars. Unless the height of the side bars relative to the height of the corrugations is correct lack of uniformity and unsatisfactory brazing of the product may result.